As far back as the Middle East oil crisis in the in 20th century, to save the energy, the high efficient condensing boiler was developed in Europe. The outstanding feature of the boiler is that the efficiency is 10% higher than the conventional boiler. Abundant water vapor in the flue is condensed and releases the latent heat of vaporization because the flue temperature can be decreased to below the dew point. This has the effect of energy saving. The condensing heat exchanger is developed and designed based on the principle of the condensing boiler.
The available heat from the combustion flue gas includes two parts: one is the sensible heat (e.g., the sensed heat) in the flue; the other part is the latent heat of water vapor in the flue. The conventional boiler has very high flue temperature because of the limitation of the structure. Therefore only sensible heat can be utilized. However, condensing boiler can not only use sensible heat in the flue but also the latent heat because of the low flue temperature. In this way, the efficiency of the condensing boiler can be greatly increased. In order to absorb the energy in the high temperature flue and collect the condensing water in the low temperature flue, a two-stage heat exchanger is employed normally. The high temperature flue enters the main heat exchanger and then condensing heat exchanger in sequence; the water flows in an opposite direction, the water enters the condensing heat exchanger first, and then enters the main heat exchanger. The water absorbs the sensible heat from the combustion flue gas after absorbing the waste heat of the high temperature flue in the condensing heat exchanger. The flue temperature decreases to a very low temperature after the sensible and latent heat in the heat exchangers has been absorbed by the water. In order to vent the flue securely, the forced convection method is applied. At the same time, the forced convection makes the boiler water absorbs the sensible and latent heat as much as possible. Therefore, the condensing heat exchanger utilizes the energy in the flue which was lost. The effect of the condensing heat exchanger depends on how much the waste energy is used.
The flue gas is normally in an overheated state before entering the condensing heat exchanger. It becomes saturation gradually as the flue temperature decreases and water vapor condenses. According to the test result, the flue at the condensing heat exchanger outlet is close to saturation status when the flue temperature is around 50° C. How close to the saturation status depends on the composition of the flue, the structure of the heat exchanger and heat transfer process. The testing result shows there are still some dead zones or short-circuit in the flue path. It decreases the heat exchanging efficiency.
The heat exchanger of the conventional (non-condensing) boiler is made by carbon steel or cast iron. The flue temperature is higher than 150° C. normally. The heat exchanger is not designed to absorb the sensible and latent heat when the water vapor condensing, and there is no condensate.
The condensing boiler is high efficient boiler with the features of energy saving and environmental protection. It is the future of the boiler industry and has been widely used. The life the condensing boiler will be shortened significantly if the carbon steel or cast iron is used because the boiler generates a lot of acid condensing water. So the material of the condensing heat exchanger should be stainless steel or cast aluminum. At present, most condensing heat exchangers are made with stainless steel tube or cast aluminum.
It is a proven technology to make cast aluminum heat exchanger, but the capacity is limited. It is very difficult to make the big cast aluminum parts. Typically, the bigger the cast, the higher the mold cost, the more complex manufacturing process and the higher scrap rate.
The efficiency can be around 96% maximally by using stainless steel or cast aluminum. However, there will be no condensing water if the return water temperature is higher than 60° C. At this point, only the sensible heat in the flue can be saved. The heat efficiency of the non-condensing boiler is only around 87%.
The conventional heat exchangers are designed according to the requirements from the different customers and the sizes of the heat exchangers are varied very much according to these requirements. Because there are a lot of components involved in the manufacture of heat exchangers, manufacturing many sizes of heat exchangers is not good practice for mass production.
While conventional heat exchangers have relatively good performance, there is still room for significant improvements in performance. A specific list of technical problems in need of improvement include: dead zones of flue flow and insufficient heat exchanging because of the poor heat exchanger structure design; increase the flue side heat transfer surface and efficiency by improving the heat transfer structure; to make the size smaller under the same heat transfer output; to integrate an air pre-heater into the heat exchanger to get the opportunity of third heat exchanging; increase the temperature of the inlet air; and decrease the flue temperature further. Accordingly, there is a need in the art to improve the heat exchanger.